Telecommunications and other applications require highly-linear filter circuits. Distortion must be kept to a minimum. This is a challenge since low-cost manufacturing demands the use of semiconductor technologies such as complementary metal-oxide-semiconductor (CMOS) integrated circuit. MOS transistors often replace resistors in these filters.
Some applications require that the filter operate and be trimmed or adjusted continuously. Variable resistors can be used in resistor networks in the filters. As line conditions, temperature, or power-supply fluctuation occur, the variable resistor can be adjusted to compensate while the filter is still operating. These filters operate on a continuos-time input and are thus known as continuous-time filters.
Capacitors can also be adjusted to provide a wider range of compensation. However, the filter must usually be halted while the capacitance is adjusted. Thus resistor tuning is preferred.
Highly-linear filters can be constructed from MOS transistors that are biased in the linear region. However, if drain voltage changes are too great, the MOS transistors may drift out of the linear region and into the saturated region. Distortion then occurs since small drain-to-source voltage changes no longer change the transistor's channel current linearly. The transistor ceases to behave as a linear resistor.
The trend towards reduced power-supply voltages exasperates this problem. For example, replacing a 5-volt power supply with a 3-volt supply reduces allowable voltage swings before distortion occurs. Transistors become saturated more easily since the gate voltage must be closer to the drain voltage. The amount of gate-overdrive is limited by the reduced power-supply-voltage available. Thus distortion caused by linear transistors entering the saturated region are more common as power-supply voltages are reduced.